The present invention relates to a compounded power MOSFET, and particularly to a compounded power MOSFET having a negative gate protection circuit for providing a negative drain withstand voltage.
Power MOSFETs are used widely for power switching devices because of their properties of small power loss, high withstand voltage and free from secondary breakdown. However, power MOSFETs have a drawback of the lack of a negative drain withstand voltage due to the presence of a parasitic diode between the drain and source. As a countermeasure against this drawback, as disclosed in Japanese patent publication JP-A-Sho-55-9444, the source is isolated from the substrate area for channel formation (will be termed "body" hereinafter) and the potential of body is externally controlled to be equal to the potential of source or drain based on the relation of potential of the drain and source.
However, the above-mentioned prior art has a problem of insufficient reduction of the on-state resistance of the power MOSFET as a result of the preclusion of the miniaturization of device due to the structure isolating the source and body. It also involves the need of external control of the body voltage depending on the source and drain potentials.